Upon brazing in a single step a tubular heat exchanger core, i.e. the circulation tubes, the dissipators or fins connected to said tubes, the tube-plates in which the ends of the tubes are fitted, the header-tanks covering the tube-plates and the lateral flanges extending on each side of the core, it is necessary to hold all the parts of the heat exchanger core under a slight pressure applied on the two header-tanks tending thus to make them come slightly nearer. The pressure must be sufficient to hold the parts together but not sufficient to cause permanent distortions thereof.
In addition, care must be taken that the expansion to which the heat exchanger is submitted, when at the brazing temperature, does not cause a permanent distortion of some parts, e.g. the tubes or the lateral flanges, said parts being respectively submitted to a differential heat expansion.
Furthermore, the connection between the tube plates and the lateral flanges must be sufficiently strong to withstand the various stresses as much as possible, when the finished heat exchanger is utilized, for example on a vehicle. Actually, this prevents noticeable mechanical stresses to be exerted on the tubes, the dissipators and the tubes and tube plates junctions which constitute the most fragile parts of a heat exchanger.
The above considerations apply to any metals or alloys utilized for making heat exchanger parts, but are of the primary importance when said parts are made of aluminium or of aluminium alloy because the brazing of said parts is performed at a high temperature, about 600.degree.C which is a temperature to which the expansion of the parts reaches significant values and to which, besides, the aluminum has poor mechanical strength as said temperature is very close to the melting temperature of aluminium.